Some large machines, particularly those associated with mining and other earthmoving operations, use dual braking systems. One configuration of such systems involves the use of an electric retarder, for example, an AC motor configured to generate electricity to slow rotation of an armature of the motor and thereby an axle coupled to the armature. As a backup to the electric motor, a conventional hydraulic braking system may also be installed in the machine. Because braking using a retarding mode in a traction motor provides a high retarding capability and reduces wear in mechanical brakes, the retarder may be the preferred system to use for ordinary braking. In the event of an electrical failure, the hydraulic braking system may also be used to slow or stop the machine.
It follows that both the electric retarder and the hydraulic braking system each have the braking capacity to bring the machine to a full stop in a worst-case operating situation, such as a mining truck with a full payload operating on a downhill grade. However, in a situation where the electric retarder is already supplying some measure of braking force and the hydraulic braking system is activated by an operator, up to double the braking force required to bring the machine to a safe stop can be applied, sometimes with several negative consequences.
First, because the electric retarder is generally mounted on an inboard portion of an axle and the hydraulic brake is generally mounted on an outboard portion of the axle or a wheel, the difference in brake torque between the electric retarder and the hydraulic brake can cause a sudden and excessive torsional shock to the driveline and axle. Second, the brake force can be so strong that the machine's pitch and bounce modes are excited and the rear wheels of the machine may actually jump off the ground and bounce causing at least stress if not damage to tires, wheels, axles, and other drive train components.
With respect to braking system performance management, U.S. Patent Publication 2012/0175200 to Ford Global Technologies discloses a system that allows a user to configure a preferred braking profile and, responsive to a brake pedal position signal, provide brake torque according to the profile. Such a system does not, however, disclose parallel braking systems or the use of a control scheme to manage over-braking.